1. Field of the Invention
The present invention relates to an envelope type inner bag for a transport tank in which cargo is contained, and more specifically, to an envelope type, inner bag whose corner portions at both ends are reinforced and the producing method thereof.
2. Description of the Prior Arts
In cargo transportation by sea, railroad, road and so forth, a tank container is generally used for liquid materials (cargo). As the tank container, a 20 foot container (hereinafter referred to as a tank container) which conforms to the ISO Standards is ordinarily used, for example. The tank container has 20 foot length, 8 foot width, and 8 foot height, so that about 20 tons of liquid can be filled therein.
In the container transportation to use this kind of tank container, it is necessary to wash the inside of the tank after transportation, and in addition, to produce the tank by using a high quality stainless steel plate with chemical resistance. In order to solve the problems, Japanese Patent Laid-Open Publication No.S61-104983 discloses that an inner bag or liner bag which is made of soft synthetic resin to have the chemical resistance is loaded in the tank produced from the general steel plate. In addition, Japanese Patent Laid-Open Publication No.2001-354292, Japanese Utility-Model Laid-Open Publication No.S61-48190, Japanese Patent Laid-Open Publication No.S50-4615, and Japanese Utility-Model Laid-Open Publication No.S57-46492 also disclose to load the inner bag in the tank in order to save the trouble for washing the inside of the tank.
However, with respect to the prior art inner bag to be used in the tanks and tank containers, it is so difficult to produce appropriate inner bags for large tanks including the 20 foot container that there has no practical application. Namely, it has been difficult to produce the inner bag fitting in the cylindrical 20 foot container easily and affordably. The ideal inner bag to fit within the tank container properly would be a cylindrical-shaped inner bag having approximately the same shape as the tank container. However, it is necessary to prepare circular lid films, and in addition, to weld the circular lid films on both ends of a tubular film. To make matters worse, since the circular lid film has to be welded not in a two-dimensional direction, but in a three-dimensional direction, the exclusive guide apparatus for welding the circular lid film is required.
In contrast, an envelope type inner bag is easily produced only by welding the both ends of the tubular film. This type of inner bag prevents the liquid from directly contacting with the inside of the tank by joining supply-discharge openings of the inner bag and the tank. Therefore, changing the inner bag makes it unnecessary to wash the inside of the tank. However, since corner portions at both ends of the envelope type inner bag are square to protrude, if filler is filled therein, the corner portions are pressed against an inner wall of the tank container. Therefore, the corner portion is rubbed against the tank due to the vibration during transporting, so that it may be damaged from the end portion of a welding line. Although the envelope type inner bag can be produced easily, strength and durability of the corner portion go down easily due to the shape in comparison with other parts, so that the practical application of the envelope type inner bag has been hampered.
Meanwhile, it may be considered to strengthen and ruggedize the corner portion by doubling the inner bag with inner and outer tubular films welded thermally at the both ends. However, mere doubling and welding at both ends make difference in thickness in the corner portions at both ends, namely four-layered portions formed by doubling and two-layered portions constituted only of the outer tubular film. When all the corner portions are welded together, the same welding energy is applied to both the four-layered portions and the two-layered portions, so that the thickness in the two-layered portions become thinner due to the application of heat. The result is a problem that the strength of the corner portion goes down.